Electrical machine and rotor for an electrical machine

ABSTRACT

An electrical machine includes a stator with a stator body supporting an electrical stator and a rotor. The rotor is supported by a bearing having a radial bearing section forming a radial gas bearing and an axial bearing section forming an axial gas bearing, the stator side parts of these bearing sections being a stator side radial bearing part and a stator side axial bearing part that are rigidly connected to one another and together form a stator bearing structure. The stator bearing structure is mounted to the other parts of the stator by either the stator side radial or axial bearing part being rigidly mounted to these other parts, and the other bearing part are connected to these other parts by an elastic support or not at all.

CROSS REFERENCE TO RELATED APPLICATION

The application is a divisional application U.S. application Ser. No.16/303,995 filed on Nov. 21, 2018, which itself is a U.S. national stageapplication of PCT/EP2017/062591 filed May 24, 2017, which itself claimspriority to EP 1617386.2 filed May 25, 2016, all of which are expresslyincorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to the field of electrical machines, in particularto high-speed electrical machines with gas bearings.

Description of Related Art

An electric motor generally includes a rotor and a stator, the statorincluding a stator body supporting and housing an electrical stator andbearings. The position of the bearings relative to the stator body canbe defined by bearing flanges of the stator body. Often, two journalbearings are present, typically located at opposite sides of the stator,as shown in the arrangement of FIG. 1 a (wherein the stator body is notshown). The precision of the alignment of the bearings in this case ismainly defined by the precision with which the bearing flange and statorbody are machined. With fluid film and in particular for gas bearings,precise alignment is crucial and this arrangement in general requiresspecial measures such as self-aligning or compliant bushing mountings,or machining, e.g. reaming, of the pair of bearings after assembly.Alternatively, the journal bearings can be arranged on the same side ofthe stator. This arrangement is often called overhanging motor design(FIG. 1 b ). With the overhanging design, the two journal bearings canbe integrated into a single part, thus precise bearing alignment iseasier to achieve. However, this approach generally results in longerrotors and therefore more critical dynamic behavior of the rotor.Furthermore, windage losses, caused by air resistance, are increased,with a negative impact on the overall motor efficiency.

U.S. Pat. No. 3,502,920 discloses a slotted electrical machine with airgap bearings, in which a bushing is located in the magnetic gap betweenthe stator and the rotor. The bushing can be elastically suspendedrelative to the stator. It defines on the one hand a radial bearing andcan include a centrally located thrust bearing or thrust block as anaxial bearing. In order to assemble the machine, the rotor needs to beseparated in the axial direction. This design is unfit for high-speedmotors.

WO 03/019753 A2 shows a spindle motor in which the rotor rotates in thestator within a thin layer of epoxy forming a cylindrical through borein the stator and serving to define both a radial bearing surface and anaxial bearing surface. The thin layer of epoxy is directly coupled tothe stator housing, and any thermally induced deformations of thehousing will immediately affect the geometry of the bearing.

US 2006/0061222 A1 and US 2006/0186750 A1 show conventional airbearings.

US 2010/0019589 A1 discloses a slot configuration of an electricalmachine, and, inter alia, a rotor having a multi-layer fiber-reinforcedcomposite sleeve wrapping. Layers can be cosmetic or have functionalcharacteristics, e.g. for achieving strength and rigidity andcontrolling thermal expansion in one or multiple directions. This can bedone by having fibers in a particular layer oriented axially, therebyproviding axial strength and limiting axial thermal expansion. Inanother layer, fibers can be oriented circumferentially, therebyproviding circumferential strength and limiting radial thermalexpansion. Thus, these layers are configured to stiffen the rotor. Theywould not be suited to carry an outer sleeve of a relatively stiffmaterial required by a gas bearing, since their purpose is to controlexpansion, i.e. limit, thermal expansion of the rotor, rather thanabsorbing differences in thermal expansion between a hard rotor core anda hard rotor sleeve.

There is a need for an electrical machine that is suited for high-speedapplications and that overcomes the abovementioned disadvantages atleast in part.

Mainly with gas bearings, the bearing member's materials are chosen tohave high rigidity and a low coefficient of thermal expansion, in orderto ensure well-defined bearing clearances under the various operatingand environmental conditions of the motor. The high rigidity of thematerials however also causes the disadvantage of high stresses in thematerial at already low strain, e.g., when combining these materialswith other materials having a higher coefficient of thermal expansion.

There is a need for a rotor that is suited for high-speed electricalmachines that overcomes the abovementioned disadvantages at least inpart.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to create an electricalmachine of the type mentioned initially, which overcomes thedisadvantages mentioned above.

It is therefore an object of the invention to create a rotor for ahigh-speed electrical machine of the type mentioned initially, whichovercomes the disadvantages mentioned above.

According to a first aspect of the invention, the electrical machineincludes a stator with a stator body supporting an electrical stator anda rotor. The rotor is supported by a bearing including a radial bearingsection forming a radial gas bearing and an axial bearing sectionforming an axial gas bearing, the stator side parts of these bearingsections being a stator side radial bearing part and a stator side axialbearing part that are rigidly connected to one another and together forma stator bearing structure.

Therein, the stator bearing structure is mounted to the other parts ofthe stator by

-   -   either the stator side radial bearing part being rigidly mounted        to these other parts, and the stator side axial bearing part        being connected to these other parts by an elastic support or        not at all;    -   or the stator side axial bearing part being rigidly mounted to        these other parts, and the stator side radial bearing part being        connected to these other parts by an elastic support or not at        all.

The abovementioned “other parts” can thus be the stator body itself oran assembly that includes the electrical stator and a carrier, with theassembly being elastically supported by the stator body.

Here and throughout this document, the terms “rigidly” and “fixed” areused as opposed to “elastically”. An elastic connection has a springrate or a Young's modulus that is at least, for example, 100 or 10,000or 1,000,000 times larger than in a rigid connection.

A rigid connection is a connection designed such that the connectedparts do not move relative to one another during normal operation of themachine. Thus, a rigid connection can be established by screwing partstogether or by pressing them against one another with a spring. In thiscase, the spring is not part of the rigid connection but provides aforce that maintains rigidity of the connection.

An elastic support can be an O-ring, typically of a (synthetic) rubber,or a metallic spring.

The gas of the gas bearing can be any gas the machine operates in, suchas air, a cooling agent, natural gas etc. The gas bearing can be apassive or an active gas bearing.

In embodiments, the stator bearing structure extends in the axialdirection of the electrical machine from a first end to a second end,and the stator bearing structure is rigidly supported by the other partsof the stator near one of the two ends and near the other end issupported elastically or not at all.

In embodiments, the stator bearing structure is attached near one end tothe stator body in a fixed manner, in particular the stator side axialbearing part is an axial bearing assembly which is rigidly mounted orattached to the stator body.

In embodiments, the stator side radial bearing part is a bushing that isrigidly mounted to the stator body or to a carrier carrying theelectrical stator.

The radial bearing part extends, in the axial direction of the machine,throughout the length of the electrical stator. It can be a single partincluding bearing elements.

The drawbacks of long rotors can thus be avoided by having the bushingwith the journal bearings arranged within the electric stator, therewithallowing short rotor designs. The journal bearings can be integratedinto a single part, thus precise alignment can be easier to achieve thanwith journal bearings on separate parts.

The end-to-end journal bearing bushing part can increase the magneticair gap of the motor and therefore have a negative impact on the motorefficiency. Although the end-to-end bushing concept is conceivable for avariety of machine types, slotless permanent magnet machines types arefavored for this bearing concept: due to the already relatively largemagnetic air gap in slotless type permanent magnet machines, theefficiency is not seriously affected.

The bushing can be made of a ceramic material or another material thatprovides sufficient mechanical stiffness and does not affect themagnetic field in the magnetic air gap. The advantage of ceramicmaterials is that they are suited both for gas bearings and can beplaced in the magnetic air gap, where they are penetrated by the torquegenerating magnetic field. Generally, electrical insulators or materialswith low thermal conductivity such as ceramics, glass ceramics ortechnical glasses, plastics, composites, mineral materials etc. can beused to avoid excessive eddy current losses caused by the alternatingmagnetic air gap field.

In embodiments, the electrical machine is of the slotless type. In otherwords, the electrical stator includes an air gap winding rather thanslotted windings. This is advantageous since it allows for a magneticair gap that can accommodate elements of an air bearing, such as abushing, without substantial disadvantages.

In embodiments, the radial bearing section extends in the longitudinaldirection of the axis of rotation and all bearing elements andcooperating rotor bearing surfaces of the radial bearing section lieoutside the magnetic gap between the electrical stator an the rotor.

In embodiments, the radial bearing section extends in the longitudinaldirection of the axis of rotation and the entirety of the radial bearingsection lies in the magnetic gap between the electrical stator an therotor.

In embodiments, the radial bearing section extends in the longitudinaldirection of the axis of rotation and at least one bearing element andcooperating rotor bearing surface of the radial bearing section lie inthe magnetic gap between the electrical stator and the rotor.

In embodiments, the radial bearing section extends in the longitudinaldirection of the axis of rotation and at least one quarter, or at leastone half, or at least three quarters of the radial bearing section liein the magnetic gap between the electrical stator an the rotor.

A thrust bearing or axial bearing typically includes a rotor disc in therotor and, at each side of the rotor disc, as seen in the axialdirection, an adjacent stator disc on the stator. The two stator discsare separated by a shim or spacer element such that a well-defined gapis formed between the rotating and stationary parts of the thrustbearing. Together, stator discs, spacer and connecting elements form theaxial bearing assembly.

In embodiments, the stator disc of the axial bearing assembly and thebushing each include an axially facing surface as an axial referencesurface and the two axial reference surfaces are placed against oneanother. Thereby they ensure that the axis of rotation is normal tobearing surfaces of the axial bearing assembly.

In embodiments, a stator disc of the axial bearing assembly and thebushing are manufactured as a single piece or part.

In embodiments, the stator disc and the bushing that are placed againstone another are pressed against one another by a resilient element. Theresilient element can be as an axial compensation element, which can bea metal spring or synthetic part such as an O-ring.

As a result, during assembly the relative axial position and the anglebetween the stator disc and the bushing is well defined by the referencesurfaces, whereas the relative position in the radial direction is notdefined. This allows assembling the parts without mechanical constraintsthat might lead to a deformation of the parts. After assembly, by beingpressed against each other the parts are rigidly connected.

For best orthogonality of journal and thrust bearings, the connectingsurface on the bushing part is manufactured precisely orthogonal to thejournal bearing rotational axis and the stator thrust disc faces aremanufactured highly parallel. The axial bearing thrust discs togetherwith the thrust bearing shim or spacer can be rigidly mounted to thestator body, e.g. by means of screws. Using the spring element (whichcan simply be an O-ring) between the stator body and the journal bearingbushing part, the bushing is orthogonally self-aligned to the axialbearing disc/shim stack or axial bearing assembly. This mounting conceptis well defined, that is, not over-determined. Thus, the stator body orits parts can thermally deform in a wide range without impact on thealignment or deformation of the bearing system.

In embodiments, two stator discs and a spacer element arranged betweenthe two stator discs are pressed against one another by a resilientelement. This resilient element can be the same one as the one thatpresses a stator disc against the bushing. There also can be tworesilient elements for pressing the two stator discs, the spacer elementand the bushing against one another. This allows for simpler assemblyand, because the system is not statically overdetermined, betterprecision with regard to axial alignment.

In embodiments, the stator bearing structure at an end at which it isnot rigidly mounted is supported by means of a first elastic support.

Such an additional elastic support on the bushing's other end allows tocompensate for a possible deformation of the stator parts and to improvevibration characteristics.

In embodiments, the stator bearing structure is thermally coupled to thestator body at or near the location of the first elastic support.

Such a thermal coupling of the bushing to the stator can be affected byusing O-rings both as flexible supporting elements and for sealing off athermally conductive paste or fluid placed in between the O-rings. Thepaste or fluid, depending on its viscosity, can have a dampening effect,as in a squeeze film damper.

In embodiments, a carrier supporting the electrical stator ismechanically decoupled from the stator body by means of elastic carriersupport elements. This allows to dampen or eliminate the transmission ofvibration between the electrical stator and the stator body.

According to a second aspect of the invention, which in principle isindependent from the first aspect but can be realized in combinationwith the first aspect, a rotor for a high-speed electrical machine isprovided. The rotor includes a rotor shaft, the shaft including a rotorcore and a rotor sleeve, an (essentially cylindrical) compensationelement being arranged between the rotor core and the rotor sleeve toabsorb differences in thermal expansion of the rotor core and the rotorsleeve.

In embodiments, the rotor sleeve, on which part also the rotatingbearing surfaces are located, is made of a ceramic material, typicallywith a low coefficient of thermal expansion. For a permanent magnetmachine, a permanent magnet can be mounted into the ceramic rotorsleeve. In general, permanent magnet materials have higher coefficientsof thermal expansion than the targeted rotor sleeve materials, thuscreating thermally induced stresses in the ceramic material whendirectly mounted or glued with a rigid adhesive.

For example, the coefficient of thermal expansion (CTE) ofSamarium-Cobalt Magnets is 9 to 13 μm/m/K, that of Neodymium-Iron-Boronmagnets is −1 to 8 μm/m/K. Both materials are anisotropic, but thecompensation element absorbs corresponding strains as well. CTEs oftypical ceramic materials for the rotor sleeve are around the range of1.5 to 5 μm/m/K.

At high rotational speeds further stresses induced by centrifugal forcesare superimposed over the thermally induced stresses.

The term “high speed electrical machine” is taken to cover machines thatare suited for more than 100,000 revolutions per minute.

Generally, it can be the case for all embodiments that the rotor sleeveis stiffer than the compensation element. The rotor sleeve can bestiffer than the compensation element with respect to a radial pressure,and in particular wherein a radial expansion of the rotor sleeve withregard to a radial pressure is at least less than half, or less than afifth, or less than a tenth of a radial compression of the compensationelement with regard to the same radial pressure. In the assembled state,a radial pressure acting between the compensation element and the rotorsleeve acts as a compressive pressure on the compensation element and asan expansive pressure on the rotor sleeve.

For thermally insensitive motor designs or for permanent magnets andceramic rotor sleeves with similar coefficients of thermal expansions,the permanent magnet can be mounted into the ceramic sleeve with a tightfit or be glued in. Thermal strain of the magnet is then directlytransferred to the ceramic sleeve, thereby limiting the maximumoperating temperature and maximum speed. In thermally more criticaldesigns and/or at higher speeds, a mechanical decoupling of the twoparts is needed.

The rotor core can include the permanent magnet fitted in a sheath inorder to improve its mechanical stability. In other embodiments, themagnetic core includes a permanent magnet which that is self-supporting,in that it does not require a sheath to maintain its stability. Ideally,the sheath should be of a material with a similar CTE than the permanentmagnet but with greater mechanical stability or toughness.

In embodiments, the compensation element includes compensation sectionsarranged to be deformed, in particular to yield or to be bent, when therotor expands or contracts due to temperature changes.

This compensation element or compensator, can be a metallic sleeve (e.g.made from titanium), with much higher elasticity than the ceramicmaterials is arranged between the rotor core (permanent magnet with orwithout a sheath) and rotor sleeve. The compensator is designed toabsorb thermally induced strain between the rotor core and the ceramicrotor sleeve and thus reduce stresses in the ceramic material. Thecompensator contacts the rotor core at least at the regions of theiraxial ends, and in other regions establishes an air gap between therotor core and the ceramic sleeve. In these regions, the rotor core andthe compensator are allowed to shrink and expand without having animpact on the ceramic sleeve. At the rotor core's axial ends, theconnection to the ceramic sleeve can be made with a well-defineddistance between

-   -   contact areas joining the rotor core to the compensator, and    -   contact areas joining the compensator to the ceramic sleeve,        along which the induced strain is relieved.

In an embodiment, when no preloading of the rotor core is needed, i.e.when the rotor core or just a permanent magnet constituting the rotorcore is self-supporting, then the compensator can include additionalpoints of support between the rotor core and the ceramic sleeve, orsupport sections of the compensation element. This can improve thedynamic behavior of the rotor. With a well-defined axial distancebetween these points of support, along which the induced strain can berelieved, the compensator contacts the ceramic sleeve with an air gaptowards the magnet, such that the magnet can shrink and expand freelywithout a noticeable impact on the ceramic sleeve.

Thus, in embodiments, the compensation element includes first sectionsin contact with only the rotor core and not the rotor sleeve, and secondsections in contact with only the rotor sleeve and not the rotor core,and compensating sections linking the first and second sections.

The second sections can include at least one flange at an end of thecompensation element, at which the compensation element has an enlargeddiameter. At the other end there can be another flange. In the remainingfirst sections the compensation element can form a tight fit with therotor core. The compensation element can be a metal and/or have at leastapproximately the same CTE as the rotor core.

In embodiments, the second sections include one or more support sectionsat one or more locations between second sections that are located atends of the compensation element.

In embodiments, the second sections include a plurality of separatesupport sections. The separate support sections are separated by hollowspaces that can include air, a gas or another substance that is morecompressible than the support sections. The separate support sectionscan be distributed along the length and around the circumference of therotor core. Separate support sections can be ring-like, extending aroundthe circumference of the rotor core, or linear, extending parallel tothe axis, or running along the rotor core in a helicoidal pattern.Separate support section can be point-like, with support sections spacedfrom one another in the axial and the circumferential direction.

In embodiments, for each point in a compensating section, a line in theradial direction passes through a hollow space before reaching the rotorsleeve and also passes through a hollow space before reaching the rotorcore. With this, the compensating sections can act as levers (when seenin a longitudinal and/or transverse cross section), being elasticallybent when absorbing differences in thermal expansion of the rotor coreand the rotor sleeve.

In embodiments, the compensating sections extend along at least one ofthe axial direction of the rotor and the circumferential direction ofthe rotor. This allows them to bend or yield in the radial direction.

Those second sections where the compensation element is attached to therotor sleeve are preferably concentrated in one region as seen in theaxial direction. In this manner rotor sleeve and compensation element atthe remaining second sections are free to slide relative to one anotherin the axial direction. For example, if there are two flanges, one eachat one of the two ends of the rotor, the sleeve is attached to oneflange and is free to slide in the axial direction on the other flangeand on any optional remaining second sections.

The same kind of distribution of attached and not attached sections canbe realized for the first sections where the compensation element andthe rotor core are in contact.

In embodiments, the compensation element is configured to be elongatedin the axial direction, thereby reducing its outer diameter from beinglarger than an inner diameter of the rotor sleeve to being smaller thanthe inner diameter of the rotor sleeve. This allows the rotor sleeve tobe assembled around the compensation element. This allows assembling theshaft by a method including the following steps:

-   -   sliding the compensation element over the rotor core;    -   applying a force for elongating the compensation element in the        axial direction, thereby reducing its outer diameter until the        outer diameter is smaller than the inner diameter of the rotor        sleeve;    -   sliding the rotor sleeve over the compensation element;    -   reducing the force elongating the compensation element, thereby        increasing the outer diameter of the compensation element such        that it pushes against the inner surface of the rotor sleeve,        thereby centering the rotor sleeve on the compensation element.

The last step also establishes a force fit between the compensationelement and the rotor sleeve.

The force for elongating the compensation element can be applied bymeans that are not part of the rotor or shaft and are removed afterassembly. Alternatively, the force can be applied and optionally alsocontrolled by elements that are part of the rotor or shaft when it is inoperation.

In embodiments, the compensation element has the shape of a corrugatedcylinder.

In embodiments, hollow spaces are formed between the compensationelement and the rotor core, and/or between the compensation element andthe rotor sleeve, and optionally the hollow spaces are ventilated byventilation openings, the ventilation openings being, for example, holesin the compensation element.

This ensures that, when the motor is to be used in an environment withan explosive gas, air (containing oxygen and posing a risk when mixedwith the explosive gas during operation of the motor) can be flushed outbefore the motor is taken into operation.

For high rotational speeds it is also possible to implement the contactbetween the rotor core and the compensator as a shrink fit. This allowsto preload the rotor core material and compensate for stresses in therotor core caused by centrifugal forces.

Instead of using a compensator in the form of a compensation sleeve torelieve the induced strains, a strain tolerant adhesive or moldingmaterial can be used. The clearance between rotor core and ceramicsleeve is filled with a material that compensates for the differentstrains of magnet and ceramic sleeve. The elasticity of the adhesive ormolding material is chosen to be low enough to yield to straindifferences of rotor core and ceramic sleeve, but still high enough inorder to prevent mechanical resonance between the rotor core and theceramic sleeve which could be excited by the rotational frequency. Thecompensation material can be silicone.

Regarding Young's modulus, achievable operating temperature anddurability, filled silicone molds are suitable materials for thecompensation material serving as a buffer between the magnet and theceramic sleeve.

Thus, in embodiments, the compensation element includes a syntheticelastic compensation material, and in particular the compensationmaterial can include a filler material for adjusting its Young'sModulus.

However, silicones have the drawback of being nearly incompressible(i.e. their Poisson ratio is close to 0.5). Therefore, high pressure isgenerated in the silicone between the magnet and the ceramic sleeve whenthe silicone expands at higher temperatures. Different measures can betaken to avoid this problem.

One approach is to include gas bubbles in the silicone which yields thecompound a more compressible behavior. The resulting (filled) siliconefoam may contain less than 1% or less than 10% or less than 30% of gasbubbles. To high gas content however will lower the overall elasticmodulus of the compound such that rotor dynamics is impaired. Apreferred method to fabricate such a rotor is to mold the magnet intothe ceramic sleeve using a filled silicone mold in combination with afoaming additive to control the gas content in the resulting compound.Another method would be to coat the magnet first with the silicone andthen shrink or press it into the ceramic sleeves.

Instead of tiny gas bubbles, the problem of low compressibility and highstresses in the ceramic rotor sleeve can be solved by a patterned orstructured silicon layer containing 1% to 50% of empty space. As anexample, the silicon layer can contain axial, ring or spiral shapedgrooves, circular cutouts or be coated in lines, circles or arbitraryshaped spots. When the magnet and silicon layer expands with highertemperature, the silicone is allowed to expand into the nearby groovesrather than build up pressure against the ceramic sleeve and producetensile stresses. The ratio of pattern width to layer thickness is inthe range of 5:1 to 20:1. Thickness is typically 1/10 mm to 5/10 mm.Preferably, the silicone layer/structure is brought on the permanentmagnet first and then the silicone coated magnet is shrunk or pressedinto the ceramic sleeve.

Another option is to first apply a (filled) silicone coating on thepermanent magnet and then roughen the coating's surface to bring in someempty space where the material is allowed to expand under increasedtemperature.

Thus, in embodiments, between the rotor core and the rotor sleeve, andadjacent to or enclosed by the compensation material, pockets of gas arepresent, increasing the compressibility of the compensation material.

Such pockets of gas can be formed by one or more of the followingmeasures:

-   -   by gas bubbles within the compensation material, or    -   by grooves in the inner and/or outer surface of the compensation        material, or    -   by roughening of the inner and/or outer surface of the        compensation material, or    -   by arranging the compensation material at disjoint locations        between the rotor core and the rotor sleeve.

In embodiments, the gas bubbles are formed by expandable hollowmicrospheres, in particular thermoplastic expandable hollowmicrospheres. These can be thermally expandable.

In embodiments, a nominal diameter of the microspheres, when expanded,is larger than a nominal distance between the rotor core and the rotorsleeve. In other words, this nominal diameter is larger than thedifference between the outer radius of the rotor core and the innerradius of the rotor sleeve. The rotor sleeve can be assembled on therotor core by the steps of

-   -   arranging the rotor core inside the rotor sleeve;    -   filling the gap between them with the compensation material        including the microspheres; and    -   causing the microspheres to expand (e.g., by thermal activation)        to their nominal diameter.

The last step causes the rotor sleeve to be centered on the rotor core,since all microspheres expand to the same nominal diameter, at least onaverage.

In other embodiments, the expandable hollow microspheres have a nominaldiameter, when expanded, that is smaller than the above nominaldistance. Then a particular microsphere does not touch both the rotorcore and sleeve.

In embodiments, the compensation element includes compensation spheresof an elastic material, arranged and thereby elastically deformedbetween the rotor core and the rotor sleeve. The rotor sleeve can beassembled on the rotor core by the step of:

-   -   sliding the rotor core inside the rotor sleeve while inserting        the compensation spheres in the gap between them; and    -   thereby elastically compressing the compensation spheres.

This causes the rotor sleeve to be centered on the rotor core, assumingthat all compensation spheres have the same diameter, at least onaverage.

In embodiments, the compensation element includes a plurality of elasticelements attached to the rotor core and deformable by sliding the rotorsleeve over the rotor core with the elastic elements, thereby aligningand centering the rotor sleeve with respect to the rotor core.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more detail inthe following text with reference to exemplary embodiments which areillustrated in the attached drawings, which show:

FIG. 1 a-b longitudinal section views of prior art machines with gasbearings;

FIGS. 2 a-c a first embodiment of an electrical machine, withvariations;

FIG. 3 an alternative elastic suspension of one end of a bushing of thefirst embodiment;

FIG. 4 a second embodiment of an electrical machine;

FIG. 5 a partial longitudinal section of a rotor with a layer ofcompensation material;

FIGS. 6 a-7 g partial longitudinal sections of rotors of a first groupof embodiments;

FIG. 7 h a transverse section of a rotor of the first group;

FIG. 8 a partial longitudinal section of a rotor with a layer ofcompensation material;

FIGS. 9-13 rotors with structured stress relieving adhesives or fillingmaterials;

FIGS. 14 a-b a rotor with a controllable compensation element.

In principle, identical parts or parts with an analogue function areprovided with the same reference symbols in the figures.

DETAILED DESCRIPTION

FIG. 1 a schematically shows a prior art electrical machine with air orgas bearings. Therein, a stator body (not shown) carries an electricalstator 3 with a coil 31 and a core 32 and further carries an axialbearing section 12 and a radial bearing section 17 in which a rotor 5with a shaft 51 and a permanent magnet 52 is arranged to rotate.Individual bearing elements 19 constituting the radial bearing section17 are arranged at opposite ends of the machine, with the electricalstator 3 in between. FIG. 1 a shows the same elements in a prior artoverhanging arrangement, with the individual bearing elements 19arranged at the same end of the machine, both at the same side of theelectrical stator 3.

Here and in the other arrangements, a fan 6 driven by the electricalmachine operating as a motor is shown as an example for an applicationof the machine. Naturally, any other end device, in particular onerequiring a high speed drive can arranged to be driven by the electricalmachine.

FIG. 2 a schematically shows a first embodiment, with the elementsalready mentioned, but in a different arrangement. In addition, thestator 1 is shown, including a stator body 25 supporting a carrier 4elastically by means of elastic carrier supports 41 such as O-rings. Theelastic carrier supports 41 serve to decouple the stator body 25 and thecarrier 4 with respect to thermal distortion. Alternatively, the carrier4 can be rigidly attached to the stator body 25, or the two can be oneand the same part. The carrier 4 carries the electrical stator 3. Thestator body 25 can be the housing of the machine or just part of it.

The parts of the axial bearing section 12 and the radial bearing section17 that are attached to the stator as opposed to the rotor form a statorbearing structure. This stator bearing structure includes thestator-side bearing surfaces of the axial bearing section 12 and theradial bearing section 17 and defines the relative position of thesesurfaces. The stator bearing structure is designed to be rigid in itselfand to be easily and reliably machined and assembled to high precision.

The axial bearing section 12 or thrust bearing section on the part ofthe rotor includes a generally disk-like thrust plate or rotor disc 54extending outwardly from an outer peripheral surface of the shaft 51near one end of the rotor 5. The rotor disc 54 has two oppositely facingaxially facing surfaces, which in cooperation with two stator discs 14,between which the rotor disc 54 is arranged to rotate, form the axialbearing.

The axial bearing section 12 on the part of the stator includes thesestator discs 14, which are part of an axial bearing assembly 11, whichfurther includes a spacer element 15, typically a washer, which definesa distance between axially facing surfaces of the of the stator discs 14facing each other, and fasteners 16 that clamp the stator discs 14 andspacer element 15 together. The same fasteners 16 can also attach theaxial bearing assembly 11 to the stator body 25.

The radial bearing section 17 or journal bearing section on the part ofthe rotor includes at least part of the outer peripheral surface of theshaft 51. This part functions as a rotor bearing surface 53, which incooperation with bearing elements 19 of a bushing 18 form the radialbearing.

The radial bearing section 17 on the part of the stator includes thisbushing 18.

In order to align the axis of rotation, which is defined by the bearingsurfaces of the bushing's bearing elements 19 of the radial bearingsection 17, to be orthogonal to the bearing surfaces of the axialbearing section 12, only a single pair of surfaces needs to be machinedwith high precision and mounted against one another: These are axialreference surfaces 13 located on an inner one of the stator discs 14 andon an axially facing surface of the bushing 18. These axial referencesurfaces 13 are machined to define a right angle between the axiallyfacing bearing surfaces of the axial bearing section 12 and the axis ofrotation.

The axial reference surface 13 of one (inner) stator disc 14 and thebushing 18 can be attached to each other by various means of fastening,including further fastening elements or welding, gluing etc.Alternatively, as shown in FIG. 2 , they can be placed against oneanother without being directly attached, but rather being pressedagainst one another by an axial compensation element 20, such as a platespring (conical spring washer) or spring washer or an O-ring, arrangedto press the bushing 18, in particular a shoulder or flange of thebushing, in the axial direction away from the stator body 25 and againstthe stator disc 14. The stator disc 14 in turn is held against thestator body 25 by the fastener 16 in a fixed manner.

Alternatively, as shown in FIG. 2 b , the stator discs 14 and the spacerelement 15 can be clamped together by means of a mounting disc 14 a anda plate spring 14 b (or O-ring which acts as spring element). The platespring 14 b clamps the stator discs 14 against the stator body 25 andalso must compensate the clamping force of the axial compensationelement 20. Thus, the plate spring 14 b must be preloaded more than theaxial compensation element 20 and is therefore generally larger andstiffer.

Another alternative is shown in FIG. 2 c . The stator discs 14, thespacer element 15 and the shoulder of the bushing 18 are clamped againstthe stator body 25. For robustness against possible tilting of thestator discs 14 relative to the bushing 18, the diameter where the platespring 14 b is in contact with the (outer) stator disc 14 can be smallerthan an outer diameter of the bushing 18, in particular of a shoulder ofthe bushing 18 that abuts the other (inner) stator disc 14.

At its other end, with respect to the axial bearing section 12, thebushing 18 is supported by the stator body 25 by first elastic supports21, for example, O-rings. This dampens mechanical oscillations thatmight arise at the otherwise free end of the bushing 18. A gap betweenthe bushing 18 and the stator body 25 can be filled with thermallyconducting filler 23. This allows dissipating heat from the bushing 18to the stator body 25.

In the embodiment of FIG. 2 , the first elastic supports 21 and theconducting filler 23 are arranged at the outside of the peripheralsurface of the bushing 18. FIG. 3 schematically shows an alternativearrangement of these elements: here the stator body 25 includes orcarries an end piece 24. The end piece 24 projects into the inside ofthe bushing 18. At least one first elastic support 21 and optionally theconducting filler 23 are arranged on the inside of the bushing 18between the bushing 18 and the end piece 24.

FIG. 4 schematically shows a second embodiment, with the elementsalready mentioned, but in a different arrangement. Again the axialbearing assembly 11 and the bushing 18 are in contact at the axialreference surfaces 13. However, in this case it is not the axial bearingassembly 11 but rather the bushing 18 that is attached to the statorbody 25 in a fixed manner. The axial bearing assembly 11 can be linkedto the stator body 25 by means of optional second elastic supports 22,for example, O-rings. Optionally, thermally conducting filler 23 isarranged in a gap between the second elastic supports 22. If the bushing18 and axial bearing assembly 11 are not joined by other means, theaxial compensation element 20, such as a plate spring, can be arrangedto press the bushing 18 in the axial direction away from a projection 11a of the axial bearing assembly 11 against the stator disc 14.Alternatively, the arrangement of FIG. 2 c can be implemented incombination with the remaining elements of FIG. 4 .

The embodiment of FIG. 4 can be implemented in combination with acarrier 4 for the electrical stator 3, supported by elastic carriersupports 41, as in FIG. 2 . In this case, the bushing 18 can be rigidlyattached to the stator body 25, with the carrier 4 and electrical stator3 remaining movable relative to the bushing 18, or the bushing 18 can berigidly attached to the carrier 4 and thereby be in a fixed positionrelative to the electrical stator 3.

In both cases, i.e. the bushing 18 being attached to the stator body 25or the electrical stator 3, the bushing 18 can be mounted by meansembedding it, in particular with a thermally conductive molding materialto obtain improved thermal coupling to the stator.

For the embodiments of both FIG. 2 and FIG. 4 , the following holds:

The bushing 18 lies within the magnetic (air) gap that separates theelectrical stator 3 and the rotor 5. Furthermore, the bearing elements19 and cooperating rotor bearing surfaces 53 of the radial bearingsection 17 lie completely or mostly within the volume through which themagnetic flux driving the motor passes.

The bearing air gap 7 lies between the bushing 18 and the rotor 5. It isnarrowest at the location of the bearing elements 19 and cooperatingrotor bearing surfaces 53, and can be wider at other locations in theaxial direction in order to reduce friction losses.

The position of the bushing 18 relative to the axial bearing assembly 11is rigidly constrained by only one mechanical link. This link is definedby the axial reference surfaces 13 on the bushing 18 and one of thestator discs 14. During assembly, these surfaces can slide on oneanother. Afterwards, they are pressed together by the axial compensationelement axial compensation element 20 and are in essence rigidlyconnected. The only other mechanical links between the bushing 18 andthe axial bearing assembly 11—via the stator body 25—are elastic orresilient since they run

-   -   via the axial compensation element 20 and the optional first        elastic supports 21 (FIGS. 2 and 3 ).    -   via the optional second elastic supports 22 (FIG. 4 )

In this way, the relative position of these parts and in particular ofthe bushing 18 with respect to the stator discs 14 is notoverdetermined. Thus, the precision of the alignment of the axial andradial bearing sections is easy to achieve, by precise machining of theaxial reference surfaces 13, and can be maintained under thermal andmechanical stress.

In other words, the axial bearing assembly 11 and bushing 18—togetherforming the stator bearing structure—and the rotor 5 can part of one ormore kinematic loops, where each loop includes at least one resilientelement. Conversely, the axial bearing assembly 11 and bushing 18 arenot part of an overconstrained loop or arrangement.

Furthermore, the location of the bushing 18, and thus the axis ofrotation, is constrained by the location of the stator body 25 in afixed manner by not more than one mechanical link, that is

-   -   via the attachment of the axial bearing assembly 11 to the        stator body 25 (FIGS. 2 and 3 ).    -   via the attachment of the bushing 18 to the stator body 25 (FIG.        4 ).

FIG. 5 schematically shows a partial longitudinal section of a rotor 5according to the prior art. The rotor 5 includes a rotor core 55 insidea rotor sleeve 56.

FIGS. 6 a through 6 c schematically show embodiments with a mechanicallyresilient decoupling of the rotor core 55 from the rotor sleeve 56 bymeans of a compensation element 57. The compensation element 57 absorbsdifferences in thermal expansion and allows to combine a rotor sleeve 56with a relatively low coefficient of thermal expansion (CTE) with arotor core 55 with a relatively high CTE.

The compensation element 57 can be made of a metal such as titanium or atitanium alloy, steel, a nickel alloy. Alternatively, it can be made ofa synthetic material such as PEEK (Polyetheretherketone), PAI(Polyamide-imide, e.g. trademarked as Torlon), etc.

The compensation element 57 includes first sections 61 in contact withonly the rotor core 55 and not the rotor sleeve 56, and second sections62 in contact with only the rotor sleeve 56 and not the rotor core 55,and compensating sections 63 linking the first and second sections.Typically, there is a tight fit or pressure fit at the first sections 61and/or at the second sections 62. Alternatively or in addition, they maybe glued. A hollow space 64 lies between the rotor sleeve 56 and thecompensation element 57. The hollow space 64 is ventilated byventilation openings 60.

In the embodiments of FIGS. 6 a and 6 b , the second sections 62 areflanges at the two ends of the compensation element 57, where thecompensation element 57 has an enlarged diameter relative to the firstsection 61. The embodiment of FIG. 6 a can be manufactured by a formingprocess. The embodiment of FIG. 6 b can be manufactured by a machiningor cutting process. In the embodiment of FIG. 6 c , the compensationelement 57 including the second sections 62 is of a substantiallycylindrical shape, without flanges having an enlarged diameter. Instead,the rotor sleeve 56 at the two ends of the compensation element 57 hasinwardly protruding elements that are in contact with the secondsections 62. At locations without such protruding elements, the hollowspace 64 is arranged between the rotor core 55 and the rotor sleeve 56.

FIGS. 7 a through 7 g schematically show embodiments with secondsections that form a support section for the rotor sleeve 56. Suchsupport sections can be provided at one or more locations in one of thearrangement of FIGS. 6 a to 6 c , or embodiments in which the rotorsleeve 56 is not supported at its ends.

-   -   The support sections can be realized as a plurality of separate        projections or bumps shaped in the compensation element 57, or        as one or more projecting ribs extending along at least part of        the rotor core 55 (FIG. 7 a ).    -   The support sections can establish a distance corresponding to a        hollow space 64 between the rotor sleeve 56 and the compensation        element 57 by an outwardly projecting element of the        compensation element 57 (FIG. 7 b ) and/or an inwardly        projecting element of the rotor sleeve 56 (FIGS. 7 c and 7 d ).    -   The support sections can establish a distance corresponding to a        hollow space 64 between the compensation element 57 and the        rotor core 55 by a cavity in the rotor core 55 (FIG. 7 c )        and/or a cavity in the compensation element 57 (FIG. 7 d ).

Whereas FIGS. 7 b through 7 d show support sections with projectionsbetween the rotor sleeve 56 and the compensation element 57 and cavitiesbetween the compensation element 57 and the rotor core 55, otherembodiments have cavities between the rotor sleeve 56 and thecompensation element 57 and projections between the compensation element57 and the rotor core 55.

FIGS. 7 e through 7 h schematically show arrangements in which thecompensation element 57 includes several separate parts or compensationparts 57 a arranged between the rotor core 55 and the rotor sleeve 56.Each of these separate compensation parts 57 a can correspond to onesupport section. The compensation parts 57 a of FIGS. 7 e through 7 gcan be manufactured by molding, in particular injection molding. Theycan be manufactured from a synthetic material or from a metal material.The compensation parts 57 a can be ring shaped, i.e. extend in acircular fashion around the rotor core 55. They can be molded separatelyand then slid onto the rotor core 55, or they can be molded in place onthe rotor core 55. This can result in the compensation parts 57 a beingstressed. Such stress can be mitigated by incorporating reinforcementrings 65 made, for example, of a metal, in particular titanium or steel,on the inside of the compensation parts 57 a where they contact therotor core 55. This is shown in FIG. 7 f.

FIG. 7 e shows ring-shaped compensation parts 57 a that can bemanufactured with a simple two-part mold without undercuts, with themolds moving in the axial direction for removing the part after molding.As seen in the longitudinal cross section, the ring shape extends in theaxial direction with an outer diameter that increases monotonously fromthe first section 61 to the second section 62, and the outer diameteralso increases monotonously from the first section 61 to the secondsection 62.

FIG. 7 g shows compensation parts 57 a with a Y-shaped cross section.X-shaped compensation parts 57 a are also possible. FIG. 7 h showsseparate compensation parts 57 a seen in the axial direction. Thecompensation parts 57 a abut one another in the circumferentialdirection. Thereby they can provide good centering of the rotor sleeve56 on the rotor core 55. The compensation parts 57 a can be manufacturedby extrusion or by (injection) molding. The compensation parts 57 a canextend in the axial direction, i.e. with the cross section of FIG. 7 hremaining unchanged at different points along the axis. Alternatively,the compensation parts 57 a can be arranged in a helix configuration. Inother embodiments, not shown, the compensation parts 57 a are notseparate but are manufactured as a single piece.

In further embodiments, not shown, projections and cavities are arrangedon the rotor sleeve 56 and/or the rotor core 55 and/or the compensationelement 57 and running in the axial direction, in analogy to theembodiments of FIGS. 7 b through 7 d where they run in thecircumferential direction.

In each embodiment corresponding to FIGS. 6 a through 6 c and 7 athrough 7 h the ribs or compensation parts 57 a or, in general, thesupport sections can be separate from one another, and/or extend in onedirection following a linear or circular or spiral trajectory. In eachcase, a hollow space 64 lies between the compensation element 57 and therotor core 55 and/or between the compensation element 57 and the rotorsleeve 56. In each case, ventilation openings ventilation opening 60(not drawn in each case) can be present as well.

For all embodiments including features of FIGS. 6 a through 6 c and 7 athrough 7 h it is the case that the rotor core 55 and rotor sleeve 56are radially decoupled. In other words, each line in the radialdirection which passes through the rotor core 55 and the rotor sleeve 56passes, in between the rotor core 55 and the rotor sleeve 56, at leastonce through a hollow space 64.

FIG. 7 a also schematically shows an optional variant in which the rotorcore 55 includes not solely the permanent magnet but the permanentmagnet 52 arranged in a sheath 59 in order to maintain mechanicalstability at high speeds. This variant can be combined with theembodiments of the other figures.

FIG. 8 schematically shows a decoupling by means of a layer ofcompensation material 58. This can be an adhesive or filling materialwhich accommodates the different CTE's of the rotor core 55 and rotorsleeve 56, and thereby relieves corresponding stress.

The compensation material 58 can be silicone, to which filling materialscan be added in order to determine its Young's modulus to a desiredvalue. Such values can be 5 to 50 MPa. Filling materials can be ceramicparticles with sizes of less than 50 micrometers. Unfilled silicone canhave values around 1 MPa or 2 MPa to 4 MPa.

FIG. 9 shows a rotor construction with a structured stress relievingadhesive and/or filling as a compensation material 58. Grooves in thecompensation material 58 provide pockets of air and increase thecompressibility of the body of the compensation material 58 as a whole,as opposed to the situation where the entire volume between the rotorcore 55 and the rotor sleeve 56 is filled with the compensation material58. The grooves are shown to run in the axial direction. With groovesrunning circumferentially, and with the grooves extending all the wayfrom the rotor core 55 to the rotor sleeve 57, the embodimentcorresponds to that of FIG. 13 .

FIG. 10 shows rotor a construction with an elastic compensation material58 including gas bubbles. Such gas bubbles can be formed, for example,by means of a foaming additive or by means of (thermally) expandablehollow microspheres 64 a. The wall thickness of such microspherestypically is so small that their mechanical behavior is like that of gasbubbles. Here the gas bubbles or microspheres have diameters that aresmaller than the distance between the rotor core 55 and the compensationelement 57.

FIG. 11 shows a rotor construction with an elastic compensation material58 including expandable hollow microspheres 64 a with a nominal diameterafter expansion that is larger than the distance between the rotor core55 and the compensation element 57. This causes the rotor core 55 andthe compensation element 57 to be automatically aligned and centeredwhen the microspheres 64 a expand.

FIG. 12 shows a rotor construction with compensation spheres 58 a of anelastic material, arranged and thereby elastically deformed between therotor core 55 and the rotor sleeve 56. The compensation spheres can beinserted in the gap between the rotor core 55 and rotor sleeve 56 duringor after assembly. The ability of the spheres to roll can be blocked byat least one of mechanically enclosing the spheres in the gap, heatingthem, coating the spheres with an adhesive before or during assembly andhardening or curing the adhesive after assembly. The hardening or curingcan be effected by at least one of heat, irradiation, ultrasound,chemical activation, waiting a certain time, etc.

FIG. 13 illustrates an embodiment in which the compensation element 57includes a plurality of elastic elements 58 b attached to the rotor core55. An outer diameter of the elastic elements 58 b is larger than theinner diameter of the rotor sleeve 56. Sliding the rotor sleeve 56 overthe rotor core 55 therefore deforms the elastic elements 58 b, aligningand centering the rotor core 55.

FIGS. 14 a-b show a rotor with a controllable compensation element 57.The compensation element 57 has the shape of a corrugated cylinder.Applying a force to the axial ends of the cylinder, pulling them apart,reduces the amplitude of the corrugation. In particular, the outerdiameter is reduced, allowing to slide the rotor sleeve 56 over thecompensation element 57. In a relaxed state, the outer diameter of thecompensation element 57 is larger than the inner diameter of the rotorsleeve 56.

Pulling the ends apart can be done with means that are part of the shaft51, as shown in FIG. 14 a : one end of the compensation element 57 has ahook-like extension 57 c that abuts a first end the rotor core 55 andlimits movement of the compensation element 57 in one direction alongthe axis. The other end has elements for pulling the compensationelement 57 in that direction by pushing against the other, second end ofthe rotor core 55. These elements can be a screw 57 a engaging threadsin the compensation element 57 and pushing against the rotor core 55 viaan axially resilient element 57 b such as a spring washer or conicalspring washer. By turning the screw 57 a, the tension force pulling atthe compensation element 57 can be adjusted, and thereby the outerdiameter of the compensation element 57 as well. After reducing thisouter diameter and sliding the rotor sleeve 56 over the compensationelement 57, the tension on the compensation element 57 can be reducedand adjusted by unscrewing the screw 57 a. The screw 57 a can be left ata certain position for setting a radial force between the compensationelement 57 and the rotor sleeve 56, or can be removed completely formaximum force.

Thus, pulling the ends apart can be done during assembly only. In theembodiment of FIG. 14 b this is done with an attachment element 57 d,such as a hole, to which an element (not shown) for pulling at thecompensation element 57 can be attached for the purpose of assembly.After assembly, the force acting between the compensation element 57 andthe rotor sleeve 56 is a function of the dimensions and materialproperties of these elements.

The compensation element 57 can be made of a metal, in particulartitanium or a titanium alloy, nonmagnetic steel, a nickel alloy, etc.Alternatively, it can be made of a synthetic material or plastic.

What is claimed is:
 1. A rotor for a high-speed electrical machine comprising a rotor shaft, the shaft comprising a rotor core and a rotor sleeve, a compensation element being arranged between the rotor core and the rotor sleeve to absorb differences in thermal expansion of the rotor core and the rotor sleeve, wherein hollow spaces are arranged between the compensation element and the rotor core, and/or between the compensation element and the rotor sleeve; wherein the compensation element is a different material than the rotor sleeve, and wherein when the rotor is viewed at a cross-section whose plane extends parallel to an axial direction of the rotor shaft and intersects a center of the rotor shaft, the compensation element extends along the axial direction between the rotor sleeve and the rotor core.
 2. The rotor of claim 1, wherein the rotor sleeve has a coefficient of thermal expansion of less than four μm/m/K or less than five μm/m/K or less than six μm/m/K.
 3. The rotor of claim 1, wherein the rotor is suited for use in a high speed electrical machine at more than 100,000 revolutions per minute.
 4. The rotor of claim 1, wherein the hollow spaces are ventilated by ventilation openings in the compensation element.
 5. The rotor of claim 1, wherein the compensation element comprises first sections in contact with only the rotor core and not the rotor sleeve, and second sections in contact with only the rotor sleeve and not the rotor core, and compensating sections linking the first and second sections.
 6. The rotor of claim 5, wherein the second sections comprise a plurality of separate support sections, and wherein the separate support sections are separated by hollow spaces containing air, a gas, or another substance that is more compressible than the support sections.
 7. The rotor of claim 5, wherein the second sections comprise at least one flange at an end of the compensation element, at which the compensation element has an enlarged diameter.
 8. The rotor of claim 5, wherein the second sections comprise one or more support sections at one or more locations between second sections that are located at ends of the compensation element.
 9. The rotor of claim 8, wherein for each point in a compensating section, a line in the radial direction passes through a hollow space before reaching the rotor sleeve and also passes through a hollow space before reaching the rotor core.
 10. The rotor of claim 5, wherein the second sections comprise a plurality of separate support sections.
 11. The rotor of claim 10, wherein the compensating sections extend along at least one of the axial direction of the rotor and the circumferential direction of the rotor.
 12. The rotor of claim 1, wherein at some of the second sections, the compensation element is attached to the rotor sleeve, and at others of the second sections, the compensation element and the rotor sleeve are not attached to one another and are able to move relative to one another in the axial direction.
 13. The rotor of claim 1, wherein the compensation element is configured to be elongated in the axial direction, thereby reducing its outer diameter from being larger than an inner diameter of the rotor sleeve to being smaller than the inner diameter of the rotor sleeve, and thereby allowing the rotor sleeve to be assembled around the compensation element.
 14. The rotor of claim 13, wherein the compensation element has the shape of a corrugated cylinder.
 15. The rotor of claim 1, wherein the compensation element comprises a synthetic elastic compensation material, and wherein between the rotor core and the rotor sleeve, and adjacent to or enclosed by the compensation material, pockets of gas are present, increasing the compressibility of the compensation material.
 16. The rotor of claim 15, wherein the compensation material comprises a filler material for adjusting its Young's Modulus.
 17. The rotor claim 15, wherein the pockets of gas are formed by roughening of the inner and/or outer surface of the compensation material.
 18. The rotor of claim 15, wherein the pockets of gas are formed by gas bubbles within the compensation material.
 19. The rotor of claim 18, wherein the gas bubbles are formed by expandable hollow microspheres.
 20. The rotor of claim 15, wherein the compensation element comprises a plurality of elastic elements attached to the rotor core and deformable by sliding the rotor sleeve over the rotor core with the elastic elements, thereby aligning and centering the rotor sleeve with respect to the rotor core. 